(a) Field of the Invention
The present invention relates to a method and an apparatus for continuously separating aromatic dialdehyde from a reaction mixture obtained by gas-phase oxidation of dimethylbenzene in the presence of a catalyst.
(b) Description of the Related Art
Among aromatic compounds, aromatic aldehydes find many uses because they have highly reactive aldehyde groups.
Aromatic dialdehydes, particularly terephthalaldehyde, are bifunctional compounds that have gained attention as an ingredient for a variety of new materials. For example, they are used in liquid crystals, conductive polymers, high-temperature engineering polymers, medicines, agrochemicals, dyes, fluorescent brighteners and specialty polymers.
U.S. Pat. No. 4,465,865 discloses a method of preparing aromatic dialdehydes in an aqueous solution that includes photo-chlorination and hydrolysis. p-Xylene or m-xylene is chlorinated at 80-90° C. by irradiation with UV to obtain several chlorinated compounds. The chlorinated compounds are mixed with an aqueous solution of hexamethylenetetramine and stirred at 100-115° C. to obtain aromatic dialdehydes. The reaction mixture including the aromatic dialdehydes is cooled and filtered to obtain aromatic dialdehydes in solid phase.
When aromatic dialdehydes are prepared in liquid phase and recrystallized by cooling, unwanted byproducts tend to be precipitated along with the aromatic dialdehydes. In such a case, the aromatic dialdehydes may be selectively separated by extraction with a specific solvent, as presented in the examples of U.S. Pat. No. 2,888,488.
U.S. Pat. No. 6,458,737 discloses a method of obtaining terephthalaldehyde along with the p-tolualdehyde byproduct by collecting a reaction gas mixture being discharged from a reactor with cooled methanol. This method is disadvantageous in that a process of separating the terephthalaldehyde from the p-tolualdehyde is required additionally.
U.S. Pat. No. 3,845,137 discloses a method of preparing multifunctional aromatic aldehydes by gas-phase oxidation. This method needs a long reaction time and is inadequate for continuous preparation of aromatic aldehydes because the aromatic aldehydes are not selectively separated from the reaction product.